The present invention relates to new highly permeable, synthetic polymeric membranes useful in membrane separations of compounds in such processes as ultrafiltration, dialysis, electrodialysis and reverse osmosis.
Such processes utilize semipermeable membranes which discriminate between solute and solvent molecules on the basis of differences in molecular size, shape, chemical structure or electric charge.
This type of membrane can also be advantageously used in gas permeation and gas diffusion.
Ultrafiltration (UF) is the term applied to the separation of relatively high molecular weight solutes and colloidally dispersed substances from their solvents. The osmotic pressure of the solute is usually negligible and plays no important part in the separation process.
Reverse Osmosis (RO) is the term normally applied to the separation of low molecular weight solutes from their solvent. In this case, the driving pressure for efficient separation must exceed the osmotic pressure of the solution.
In both UF and RO, solution under pressure flows over the surface of a supported membrane, and under the impressed pressure gradient action across the membrane, solvent and certain solute species present pass through the membrane and are collected as permeate. The solvent and solute which are retained by the membrane are referred to as the retentate.
By proper membrane selection it is possible to concentrate, purify and fractionally separate virtually any solution by simple physical means, the sole energy requirement being the compression energy of the feed liquid. This is particularly appealing to manufacturers of thermally and unstable products where traditional separation process, such as evaporation, selective extraction and selective precipitation, often lead to product loss or deterioration.
Membranes commonly used to date for UF are so-called anisotropic membranes, originally developed by S. Loeb and S. Sourirajan at the University of California, Los Angeles in the late 1950's. These membranes are made or "cast" from a solution of a polymer in a solvent (e.g. cellulose acetate dissolved in acetone media). A thin layer of the solution is spread onto a suitable surface, such as a glass plate, and the solvent is allowed to evaporate to the extent that a semi-solid matrix is formed with a skin surface layer, which is due to the fact that the surface layer dries faster than the layers underneath. The membrane is then quenched in another solvent, usually water-based, to rapidly precipitate the remaining polymer; the rapid precipitation or coagulation of polymer forms the sponge like backing of the membrane.
The resultant membrane is an extremely thin layer or film of very fine pore texture polymer (&lt;S.mu. thickness) supported by much thicker layer of highly porous material (thickness&gt;100.mu.). In such membranes only the surface layer or film is active in UF. Further, because the rate of flux through such membranes is low, UF processes utilizing such membranes consume relatively large amounts of energy, are time consuming, and require relative high capital investment in plant or equipment to ensure that use of the membrane is economic or practical, in terms of the rates of flux obtainable from such membranes.
More recently, various types of membrane have been developed using polyelectrolyte, polysulfone and polycarbonate, particularly by such companies as Amicon Corporation and Dow-Oliver (U.S.A.), Sartorius and Gelman (West Germany) and DDS (Denmark). These membranes have either a skin, as described above, or a regular sponge texture. Furthermore, the General Electric Company has developed a technique for manufacturing extremely thin membranes with holes created by nuclear bombardment, and in this case the membranes have a structure which is equivalent to that of a monoscreen. From 1965 to 1970 Du Pont (U.S.A.) and OPI (France) have developed polyamide membranes, either in the flat or the "hollow filter" form. Due to limited permeability and "skinned" properties these membranes where not considered as different in their practical use to classical membranes.
For all prior art membranes, only the surface of the membrane is active and it is purely the property of the active side in contact with the liquid which gives the rejection characteristic of the membrane.
Because of the structure of the prior art membranes, the UF flux is limited in most cases by the formation of a gel layer, external to the membrane, which gel layer is constituted by the species stopped by the membrane. The permeability and cut-off characteristic of the gel layer determine the performance of the membrane. For this reason the flux of permeate through the membrane even at low pressure becomes independent of the pressure drop through the membrane. Further, the flux of permeate is strongly dependent on the wall shear rate, and high flux can only be obtained with the aid of expensive pumping devices to establish a sufficient velocity of fluid in contact with the membrane, to minimise the effect of the gel layer. Furthermore, the flow rate decreases rapidly when the concentration is built up. Because of this effect UF is unattractive or impractical for removal of solvent from highly concentrated solutions.
Another consequence of the structure of the classical prior art membrane is the extremely low rate of flux of membranes having a low molecular weight (M.W.) cut-off. This factor of water permeability is very often the overall limiting factor for membrane use.
Furthermore, the realisation of known types of membranes needs very high manufacturing quality control of the structure of the surface of the membrane which results in high manufacturing costs.